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ADP3300 Datasheet(PDF) 7 Page - Analog Devices |
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ADP3300 Datasheet(HTML) 7 Page - Analog Devices |
7 / 8 page ADP3303A –7– REV. A The proper formula to compute R1 and R2 is: R kV R k V SEL SEL 1 44 1 189 2 44 1 1 189 = × = − ΩΩ . , . Where V SEL is the desired output voltage. The output voltage can be selected from 2.2 V to 10 V. R1 is connected from the OUT pin to the FB pin and R2 is connected from the FB pin to GND. As an example, the Feedback Resistor Selection Table shows the feedback resistor values for 3 V and 5 V output voltages. Table I. Feedback Resistor Selection Table R1 R2 V OUT (1% Resistor) (1% Resistor) 3 V 110 k Ω 73.2 k Ω 5 V 187 k Ω 57.6 k Ω OUTPUT CURRENT LIMITING Short circuit protection is provided by limiting the pass transis- tors base drive current. Maximum output current is limited to 200 mA. THERMAL OVERLOAD PROTECTION The ADP3303A is protected against damage due to excessive power dissipation by its thermal overload protection circuit, which limits the die temperature to a maximum of 165 °C. Under extreme conditions (i.e., high ambient temperature and power dissipation), where die temperature starts to rise above 165 °C, the output current is reduced until the die temperature has dropped to a safe level. The output current is restored when the die temperature is reduced. Current and thermal limit protections are intended to protect the device against accidental overload conditions. For normal operation, device power dissipation should be externally limited so that junction temperatures will not exceed 125 °C. CALCULATING JUNCTION TEMPERATURE Device power dissipation is calculated as follows: PD = (VIN – VOUT) ILOAD + (VIN) IGND Where ILOAD and IGND are load current and ground current, VIN and VOUT are input and output voltages, respectively. Assuming ILOAD = 200 mA, IGND = 4 mA, VIN = 5.5 V and VOUT = 3.0 V, device power dissipation is: PD = (5.5 V – 3.0 V ) 0.2 + 5.5 × 0.004 = 0.522 W The proprietary thermal coastline TSSOP-14 package of the ADP3303A, in conjunction with the recommended PCB layout shown in Figure 21, yields a thermal resistance of 96 °C/W. As a result, the die temperature rise for the example circuit is: ∆T = T J – T A = PD × θJA = 0.522 × 96 = 50.1°C If the maximum ambient temperature is 50 °C, this yields a maximum junction temperature of TJMAX = 100.1°C, which is below the 125 °C maximum operating junction temperature rating. PRINTED CIRCUIT BOARD LAYOUT CONSIDERATION The rate at which heat is transferred is directly proportional to the temperature differential between the die and PC board. Once heat is transferred to the PC board, it should be dissipated to the air or other medium. Surface mount components rely on the conductive traces or pads to transfer heat away from the device. Appropriate PC board layout technique should be used to remove heat from immediate vicinity of the package. The following general guidelines will be helpful when designing a board layout: 1. PC board traces with larger cross section areas will remove more heat. For optimum results, use PC’s with thicker cop- per and or wider traces. 2. Increase the surface area exposed to open air so heat can be removed by convection or forced air flow. 3. Do not solder mask or silk screen the heat dissipating traces. Black anodizing will significantly improve heat reduction by means of increased radiation. Figure 22 shows the recommended board layout for the ADP3303A. Although it is not critical, make sure R1 is con- nected right at the pin or the point you want to regulate in order to realize a proper kelvin connection. This will improve overall precision and stability. The same consideration is valid for the R2 connection to the ground pin, but a short connection is strongly suggested. No other components can be connected to the FB pin except an optional 10 nF–100 nF capacitor (C NR) in parallel to R1 that serves as a noise reduction capacitor. SHUTDOWN MODE Applying a TTL high signal to the shutdown pin, or tying it to the input pin, will turn the output ON. Pulling the shutdown pin down to 0.3 V or below, or tying it to ground, will turn the output OFF. In shutdown mode, quiescent current is reduced to less than 1 µA. INPUT–OUTPUT DROPOUT VOLTAGE AND DROPOUT DETECTOR The ADP3303A maintains a regulated output with an input voltage as low as 150 mV above the nominal output voltage. Input voltage falling below this level will generate an error signal indicating that the error amplifier output is reaching its satu- rated state and will not be able to drive the pass transistor any harder. Lowering the input voltage any further will result in output voltage reduction and loss of regulation. The input voltage threshold which generates the error output signal depends on the load current. At the rated output current, it is slightly lower than the nominal output voltage plus the dropout voltage. However, the threshold is much lower at lighter loads. APPLICATION CIRCUITS Crossover Switch The circuit in Figure 23 shows that two ADP3303As can be used to form a mixed supply voltage system. The output switches between two different levels selected by an external digital input. Output voltages can be any combination of volt- ages from the Ordering Guide. |
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